1. Field of the Invention
This invention relates to a digital modulating apparatus and a digital demodulating apparatus used for recording a PCM audio signal, digital data used in a computer, etc., on an optical disc or other recording medium.
2. Description of the Prior Art
In order to record digital data on a magnetic tape, optical disc or other recording medium, the digital data to be recorded is modulated. This modulation is called digital modulation or channel coding. There has been proposed various methods of digital modulation. Parameters of evaluation of digital modulating methods involved window margins Tw, minimum length between transition Tmin, maximum length between transition Tmax, recording density DR (density ratio), and so on. Digital modulating methods that have been proposed heretofore are directed to removal of a DC component in a frequency spectrum of a modulated signal (so-called DC free) and an increase of the minimum length between transition Tmin or an increase of the window margin Tw.
For example, EFM (eight to fourteen modulation) employed in a CD (compact disc) converts 8 data bits into 14 bits as channel described in Japanese Patent Laid Open Publication No. Sho 57-48848. Selected are bit patterns of 14 bits which include two or more "0"s (logic 0) sandwiched between "1" (logic 1) and "1". Bit patterns which satisfy this condition include 277 patterns from among (2.sup.14 =16,384) patterns, and these 277 include 267 patterns whose Tmax's do not exceed a predetermined value. 256 patterns among these 267 patterns are associated with 256 8-bit data bits by one to one.
When indicating the data bit interval by Tb, the above-mentioned EFM has the parameters indicated below: EQU Tw=(8/17) Tb EQU Tmin=3Tw=(24/17) Tb EQU Tmax=11Tw=(88/17) Tb EQU DR=(24/17)
Respective symbols of 14 bits satisfy the condition that two or more "0"s are placed between "1" and "1". In order to satisfy this condition also between symbols. 3-bit coupling bits are used. Four such coupling bits are (000), (100), (010) and (001). One of four coupling bits is determined so as to satisfy the parameters Tmin, Tmax, etc., and to minimize DSV (digital sum value: where the summation is done regarding high levels as 1 and low levels as 1) of data after modulation. By deciding the coupling bit according to this rule, low frequency components of the modulated data can be reduced. Therefore, data bits in the form of 8 bits are finally converted into channel bits in the form of 17 bits.
The prior art digital modulating method for converting M bits into N bits has been performed by an arrangement which preliminarily obtains conversion tables using a computer, etc., then enters data of M bits in ROM or PLA storing the tables, and then obtain a coded output of N bits or (N-p) bits (where p is the bit number of coupling bits). Therefore, the data converting portion occupies a large part of the circuit arrangement and prevents scale reduction of the circuit.